Field of the Invention and Related Art Statement
The present invention relates to a circuit protector for disconnecting a circuit from a D.C. power supply source when an overcurrent flows through the circuit.
Among various types of known circuit protectors, there has been developed an oil-dash-pot type circuit protector. The circuit protector of this oil-dash-pot type comprises electromagnetic tripping-off means having a case containing a silicon oil, a coil wound around the case, an electromagnetic pole provided at one end of the case, a movable iron piece arranged outside the case movably so as to be magnetically attracted by the electromagnetic pole, and a magnetic plunger movably arranged inside the case and energized in the direction apart from the electromagnetic pole by a return spring provided in the case, wherein the plunger is moved and made into contact with the electromagnetic pole against the damping action of the return spring and silicon oil by the magnetic force induced between the pole piece and plunger when an overcurrent flows through the coil, thereby attracting the movable iron piece to the electromagnetic pole, and interrupting an electric path by separating a movable contact from a fixed contact in conjunction with the displacement of the movable iron piece.
In this circuit protector, however, the electric path is interrupted by moving the plunger against damping action of the return spring and silicon oil in generation of an overcurrent and making it into contact with the electromagnetic pole, so that it takes a relatively long time from the generation of the overcurrent to the actual interruption of the electric path. As shown by curves A and B in FIG. 1 illustrating response characteristics of the known circuit protectors of oil-dash-pot type, the response time is not only long, but also fluctuates over a wide range. Therefore, the known circuit protector has a low reliability. Further, since the magnetic resistance of the magnetic circuit for a magnetic flux induced by the coil is extremely large and the plunger has to be moved against the relatively large damping force of the return spring and silicon oil, the large flux density is required, and magnetomotive force should be made large by increasing the number of coil turns, and hence an internal impedance of the circuit protector becomes disadvantageously large and the circuit to be protected is affected to a large extent, because the circuit protector is connected in series with the circuit across the D.C. power supply source and a very large voltage drop is produced across the circuit protector.